One type of conventional input scanning system for recording electronically the information content of a segment of a document utilizes a long, linear array of solid state photosensitive detectors and a multi-element lens system which conveys to the linear detector array variations in the light reflectance of the document segment as the document segment is scanned by a focused, high intensity light beam, such as a laser beam. To provide acceptable performance, the lens system is comprised of an array of three or four high quality lens elements which add substantially to system cost. Also, the lens array allows for only very slight variations in the depth of field of the system which may produce imaging errors if the document is wrinkled or otherwise has slight fluctuations in movement toward or away from the lens system.
Regarding the detector array, charge coupled devices are one type of photosensitive device which can be utilized as the elements of the detector array. When the bit density per unit length of the document segment is large (about 500 bits/inch) and the document is several unit lengths wide (standard paper width), the number of charge coupled devices that must be provided in a unitary, linear array is greater than can be provided by conventional semiconductor fabrication technology. For example, 7000 charge coupled devices on 0.51 mils centers would require an approximately 3.5 inch wafer of silicon--a formidable production task requiring unlikely breakthroughs in semiconductor fabrication technology to achieve acceptable yields. To overcome this problem, it has been proposed that several short, linear charge coupled device arrays be aligned along a single axis to provide the effect of one array long enough to provide for all of the bit storage needed. Implementation of long linear charge couple device arrays from a plurality of shorter linear charge couple device arrays requires that a crossover from one array to the next occur at some position along the scan line. Aligning the arrays, for example, so that the last detector of the previous array and the first detector of the next array lie on common centers, requires sensitive opto-mechanical tolerances which could be costly, difficult to maintain in a machine environment, and inconvenient to implement on a production basis. Furthermore, satisfactory electronic techniques are not now available for removing the tangential and sagittal misalignments inherent in the construction of a single, long linear array of detectors from an assembly of a plurality of short linear detector arrays.